1. Field of the Invention
The present invention provides a method of improving the flatness of a microdisplay surface and a liquid crystal on silicon (LCoS) display panel and method of manufacturing the same, and particularly an LCoS display panel with a good liquid crystal arrangement to improve optical performance.
2. Description of the Prior Art
LCoS (liquid crystal on silicon) display technology is the key to reflective LCD projectors and rear-projection televisions. The LCoS displays have the advantages of tiny size, high resolution, low power, low cost, etc. The difference between an LCoS display and an TFT-LCD is that the TFT-LCD uses a glass substrate or a quartz substrate as the back plane and is provided with a light source from the back side, while the LCoS display uses a silicon substrate as the back plane and is provided with a light source from the front side. As a result, the manufacturing of LCoS display panels can be integrated into standard semiconductor processes, and therefore has high stability and resolution.
However, in conventional LCoS display panels, the liquid crystal molecules in the liquid crystal layer tends to be toppled due to the recess between the mirror layer units. Referring to FIG. 1, a semiconductor substrate 12 comprises transistor devices (such as CMOS), metal interconnects, and metal plugs (not shown). A plurality of reflective mirror layer units 14 are formed on the surface of the semiconductor substrate 12. A dielectric layer 16 is filled in the gap between the mirror layer units 14. A transparent electrode 22 and a transparent substrate 24 are combined with the semiconductor substrate 12. Liquid crystal layer 26 is filled in the cell gap between the transparent electrode 22 and the semiconductor substrate 12. It is noted that a surface on which the liquid crystal layer 26 stands is comprised of a reflective mirror layer 14 and the dielectric layer 16. The surface has a recess at the position of the dielectric layer 16 due to the process. The liquid crystal molecules between the two pixel electrode areas are toppled due to the recess, resulting different arrangement angles for the liquid crystal molecules. Thus, the optical performance for the location between two pixels is affected.
Refer to FIG. 2 illustrating a conventional method of manufacturing LCoS display panels. In the conventional method, after reflective mirror layer units 14 are defined, a dielectric layer 16a is deposited to fill the gaps between the mirror layer units 14. Due to processes, a recess 18 occurs on the surface of the dielectric layer corresponding to the gaps between the mirror layer units 14. Thereafter, referring to FIG. 3, a CMP process is generally used to remove part of the dielectric layer, and an etching process is continued to partially remove the resulting dielectric layer 16a, for not damaging the mirror layer units 14 (generally, aluminum). To ensure entire removal of the dielectric layer on the mirror layer units 14, an over etching is performed to leave the dielectric layer 16 only in the gaps; and the recess 20 is accordingly formed. The recess of the dielectric layer in the gaps is caused by process uniformity, similar to the inter layer dielectric (ILD) planarization in a general IC process. The recess 20 may have a depth, R, of more than 500 Å, and the arrangement of liquid crystal is affected.
U.S. Pat. No. 6,569,699 discloses a method of fabricating a LCD-on-silicon pixel device, in which, a pixel electrode is made by forming a first opaque conductive layer (comprising for example W, Ti, TiN, Cr, Ag, Co, or CoN) over the ILD, completely filing the via, planarizing the surface of the first opaque conductive layer having the recess caused by the plug, forming a second opaque conductive layer (comprising for example aluminum (Al), an aluminum copper alloy (AlCu), or aluminum silicon copper alloy (AlSiCu)) over the first opaque conductive layer, defining the pixel electrodes, and forming an optical interface layer as a reflective layer (for example, ON (i.e. an oxide layer over a nitride layer), ONON, ONONON, etc., up to five ON pairs) on the pixel electrodes. This patent intends to improve the light reflectivity of the pixel electrodes, while gaps exist between the pixel electrodes.
U.S. Pat. No. 6,750,931 discloses a reflective liquid crystal device, in which, four dielectric layers having different refractive indices are formed in order directly on the pixel electrode surface to improve light reflectivity. The problem of a recess on the surface on which liquid crystal molecules stand is not mentioned.
Udayan Ganguly and J Peter Krusius in “Fabrication of Ultraplanar Aluminum Mirror Array by Novel Encapsulation CMP for Microoptics and MEMS Applications,” Journal of The Electrochemical Society, 151 (11) H232-H238 (2004), disclose a method of manufacturing pixel electrodes of a micro-display by encapsulation CMP. Unlike a general technique to forming a metal layer on the planar ILD to make pixel electrodes, in this method, a plurality of trenches are formed by etching the ILD, a metal layer is deposited conformally in the trenches, and a cap layer is deposited conformally over the metal layer, similar to encapsulate the metal layer in a plurality of trenches. Next, a part of the cap layer around the die is removed by reactive ion etching (RIE), and then the higher portion of the cap layer on the trench wall is removed by CMP. Finally, the part of the metal layer located on the trench wall higher than the bottom of the trench is removed by CMP, to achieve a planar surface. The surface comprises a cap layer and a small portion of the metal layer revealed from the cap layer.
Therefore, a LCoS display panel and the manufacturing method to solve the problem of the recess of the surface on which the liquid crystal molecules stand in order to improve optical performance of the display device are still needed.